Atherosclerosis is the leading cause of death in the developed world, and atherosclerosis is predicted to be the leading cause of death in the developing world in the 21st century. Liver X receptors (LXRs) are ligand-activated transcription factors that play a crucial role in regulating the expression of genes involved in lipid metabolism and cellular cholesterol homeostasis. LXR agonists have been shown to enhance reverse cholesterol transport (RCT), facilitating cholesterol trafficking from the periphery back to the liver for processing and excretion. RCT occurs via upregulation of cholesterol transporters (ATP-Binding Cassettes: ABCA1 and ABCG1) in peripheral macrophages. Active RCT has the potential to inhibit the progression of atherosclerosis.
There are two isoforms of LXR, LXRα (NR1H3) and LXRI3 (NR1H2), encoded by separate genes. LXRα expression is tissue-selective, detectable in liver, intestine, kidney, adipose tissue and adrenal glands, all of which are important for lipid homeostasis, whereas LXRβ is expressed ubiquitously. Both LXRs require the retinoid X receptor (RXR) as an obligate heterodimer partner to recognize and bind cooperatively to LXR response elements (LXREs) consisting of two direct repeats of a core hexameric sequence spaced by four nucleotides (DR4). The ligand binding domains of the two LXRs are fairly well conserved (˜78% amino acid homology) and respond to endogenous ligands consisting of oxidized derivatives of cholesterol (oxysterols) that serve as intermediates in steroid hormone and bile acid synthesis. The most potent of such endogenous ligands are 22(R)-hydroxycholesterol, 24(S)-hydroxycholesterol and 24(S), 25-epoxycholesterol. These data suggested an important role for LXRs in cholesterol regulation, which was later confirmed through gene knock-out studies in mice. Non-steroidal ligands have also been identified, and, using these as chemical probes many LXR-regulated genes have been discovered. Several LXRE-containing genes are involved in cholesterol metabolism, reverse cholesterol transport (RCT) and lipogenesis. Other genes involved in inflammation and carbohydrate metabolism lack LXREs, but are repressed by LXRs in a ligand-dependent manner. Based on these discoveries, the liver X receptors have recently emerged as unprecedented targets acting as intracellular cholesterol sensors, providing the basis for the treatment of a variety of diseases, including atherosclerosis, diabetes, Alzheimer's disease, skin disorders, reproductive disorders and cancer (Viennois et al., 2011, Expert Opin. Ther. Targets, 15 (2):219-232; Hong et al., 2014, Nature Reviews Drug Discovery, 13:433-444). Additionally, it has been determined that LXR agonists modulate intestinal and renal sodium phosphate (NaPi) transporters and, in turn, serum phosphate levels (Caldas et al., 2011, Kidney International, 80:535-544). Thus, LXR is also a target for kidney disorders, and particularly for the prevention of hyperphosphatemia and associated cardiovascular complications. Recently, LXRs have been identified as targets in the treatment of osteoporosis and related diseases (Kleyer et al., 2012, J. Bone Miner. Res., 27 (12):2442-51).
Alzheimer's disease is one of the most common forms of dementia, characterized by the accumulation and deposition of amyloid-beta (Aβ) peptides in the brain, leading to the perturbation of synaptic function and neuronal loss in the brains of affected individuals. Neurons in the brain produce Aβ peptides via cleavage of amyloid precursor protein (APP), and Aβ peptides are normally cleared through efflux into the peripheral circulation and by degradation by proteinases within the brain.
Apolipoprotein E (apoE) is associated with age-related risk for Alzheimer's disease and plays critical roles in Aβ homeostasis. LXR increases the expression of apoE and increases the lipidation of apoE. Degradation of Aβ both intra- and extracellularly is enhanced by lipidated apoE. LXR agonist treatment stimulated proteolytic degradation of Aβ, reduced plaque pathology, and improved memory in APP-expressing transgenic mice (Jiang et al., 2008, Neuron, 58:681-693).
In skin, keratinocytes are a critical component of the epidermis. The outer layer, stratum corneum, is primarily responsible for the permeability barrier to water and electrolyte transit. Keratinocytes in the epidermis undergo differentiation which culminates in keratinocyte cornification (“the bricks”) and in formation of the extracellular lipid-enriched lamellar membranes (“the mortar”) in the stratum corneum. Both LXRα and LXRβ are expressed in keratinocytes, and LXR expression and activation promotes epidermis barrier function. Activation of LXR is involved in keratinocyte differentiation, formation of the lamellar membrane and overall improvement of epidermal barrier function. Thus, LXR activation is expected to result in increased keratinocyte differentiation, increased lipid secretion (via ABCA1, ABCA12), and increased lamellar body formation, leading to a healthy epidermis (smooth skin).
The potential therapeutic utility of LXR agonists has led to the development of several high affinity LXR ligands with potent agonism for both receptor subtypes. The therapeutic utility of LXR agonists is constrained by their potential to induce lipogenic genes including sterol response element binding protein-1c (SREBP1c) and fatty acid synthase (FAS). Preclinical studies have demonstrated that synthetic modulators of LXRs reduce lesion progression in murine models of atherosclerosis with limited increase in hepatic lipogenesis. There is a clear need for new LXR chemotypes that retain the anti-atherosclerotic efficacy of current LXR agonists but are devoid of lipogenic activity. Compounds exhibiting a pharmacological profile with positive effects on RCT while being neutral or suppressive on lipogenic genes will be valuable therapeutic agents in patients with atherosclerotic dyslipidemia.
Rett Syndrome (RTT) is an X-linked neurological disorder presenting with autistic features that afflicts approximately 1 in 10,000 females. Mutations in the X-linked gene, methyl CpG binding protein 2 (MECP2), are the primary cause of RTT. Hemizygous males with truncating or loss-of-function mutations typically die of encephalopathy, whereas mild mutations in either sex are associated with a variety of intellectual disabilities. Approximately 80% of RTT clinical cases show a typical clinical picture, characterized by loss of acquired cognitive, social, and motor skills in a typical four-stage neurological regression, together with development of autistic behavior. Recently, researchers showed that cholesterol metabolism is perturbed in brains and livers of Mecp2-null male mice, and inhibitors of cholesterol biosynthesis (statins) ameliorate the systemic imbalance of lipid profile, alleviate motor symptoms and confer increased longevity in Mecp2 mutant mice, suggesting that cholesterol homeostasis maintenance could be altered in patients affected by RTT (Buchovecky et al., 2013, Nat. Genet., 45:1013). These findings suggest that the disease may be ameliorated or even reversed by genetic or pharmacological means after symptom onset. Since LXR agonists have the capacity to actively remove cholesterol from peripheral tissues, leading to elimination of cholesterol from the body, LXR agonists are useful for the treatment of patients suffering from RTT.
Suppression of LXR activity in liver has been proposed for the treatment of hepatic diseases, such as fatty liver, cirrhosis, nonalcoholic fatty liver disease (NAFLD), and non-alcoholic hepatosteatosis (NASH) (Ducheix et al., 2013, Biochem. Pharmacol., 86:96-105). LXR antagonists have been shown to down regulate lipogenic genes in liver, limit hepatic accumulation of lipids, and reduce plasma cholesterol levels in a mouse model of NASH (Griffett, et al., 2013, ACS Chem. Biol., 8:559-567), and to reduce plasma levels of total cholesterol, triglycerides, and free fatty acids in mice fed a high fat diet (Jwa et al., 2012, Biochem. Pharmacol., 84:1501-1510). Therefore, liver-specific LXR antagonists are expected to be useful in the treatment of metabolic disorders associated with fat accumulation in the liver. Activation of LXRα has been shown to induce lipid synthesis and sebum secretion in sebocytes (Hong et al., 2008, J. Invest. Dermatol., 128:1266-1272). Given that excess sebum production is a major cause of acne, LXR antagonists are expected to have therapeutic potential in the treatment of sebaceous gland-associated disorders such as seborrhea and acne. Thus, antagonism of LXR has therapeutic potential in the treatment of diseases such as seborrhea and acne, as well as hepatic diseases such as cirrhosis, NASH, and NAFLD, and there is a need for new LXR modulators that antagonize LXR.